Sound wave transducer



Dec. 18, 1951 J; ANDERSON 2,579,136

SOUND WAVE TRANSDUCER Filed June 30, 1949 AMPL lF/Ef? fl/VD 070/6473)? 0575470,?

Inventor Lew/22? 'clAzzderJoiz (Ittorueg Patented Dec. 18,1951

SOUND WAVE TRANSDUCER Leslie J Anderson, Moorestown, N. J assignor to Radio Corporation of America, a corporation of Delaware Application June 3| 1949, Serial No. 102,228

'7 Claims. (Cl. 313-446) wave transducers, and particularly to an improved frequency sensitive sound wave transducer in the form of an electronic tube.

There are many instances in which sound waves, and particularly so-called ultrasonic (high frequency sound) waves, are extremely useful for industrial and military applications. For example, systems are well known for transmitting intelligence by means of modulated sound waves, for detecting obstacles by means of reflected sound waves, and the like. In such systems, it is customary to provide a transducer at the point of wave reception which will convert the sound waves into electrical energy for more convenient handling from the standpoint of amplification and the like. Usually, it is also necessary to provide some kind of mechanical or electrical filtering system in order to separate the desired sound waves from other interfering waves. Such electrical and/or mechanical filters are often relatively complex to design and manufacture, and represent an additional item to complicate the apparatus.

It is a principal object of my invention to provide an improved transducer for converting sound waves of predetermined frequency into electrical energy.

Another object of my invention is to provide a combined filter and sound wave transducer.

Another object of my invention is to provide a novel electronic tube.

According to my invention, the foregoing and other objects and advantages are attained by mounting a so-called free-free bar in one wall of an electronic tube envelope so that one of the free bar ends can serve as one of the tube electrodes. The term free-free bar is used herein and in the appended claims to designate an elongated, vibratory element supported approximately at its center and adapted to'vibrate transversely at a frequency determined by the bar dimensions with a nodal point at the bar center and antinodal points at or near the bar ends when either bar end is set in motion transversely. By mounting such a bar in the tube envelope wall as described, the electron stream in the tube will be subject to variation due to variations in the spacing being the bar electrode and the other tube electrode or electrodes. The bar will vibrate in response to sound waves of predetermined'frequency impinging on the free bar end outside the tube envelope.

A more complete understanding of the invention can be had by reference to the following description of illustrative embodiments thereof when considered in connection with the accompanying drawing wherein:

Fig. 1 illustrates vibratory motion of a freefree bar of the type involved in my invention,

Fig. 2 is a perspective view of a transducer arranged in accordance with my invention, together with a schematic and block diagram of a typical network connected thereto.

F s. 3 and 4 illustrate modified forms of the transducer shown in Fig.2.

Referring to the drawing, in Fig. 1 there is shown a free-free bar It comprising a fiat elongated element supported at its center by a member l2 through which the bar In extends. It can be shown that, if either end of the bar I!) is struck or otherwise displaced (vertically, in the case of the bar shown in Fig. l), the bar will vibrate in a pattern substantially as shown by the lines Ha, llb. It can also be shown that frequency of vibration in the mode illustrated is given by the formula l=length of the bar III in cm. Q=Youngs modulus in dynes/cm. K=the radius of gyration of the bar section. =density of the bar in grams/cubic cm.

In accordance with my invention, the principle illustrated in Fig. 1 is utilized to provide a frequency sensitive sound wave transducer of simple construction.

Referring to Fig. 2, a sound wave transducer T illustrating my invention comprises an electronic tube, having an evacuated metal envelope it provided with a base member I8, and with prongs 20 in the base I 8 for connecting the internal tube electrodes to en external circuit. A cathode electrode 22 is mounted within the envelope l6, and a free-free bar I0 extends through and is supported by the end wall Ilia of the tube envelope l6 so that one end Illa of the bar I0 is disposed within the tube envelope adjacent the cathode 22 to serve as an anode electrode, while the other end lllb of the bar I0 is disposed outside the tube envelope IS. The line of support lllc for the bar In is approximately at the center thereof.

The cathode 22 may be either of the directly or indirectly heated type, supplied with heating current from a battery '24 or the like. For simplicity, no details are shown as to the manner of heating or mounting the cathode 22 since such details are all well known in the electronic tube art. As shown. the tube envelope I6 is preferably made of metal to simplify electrical connection between the bar II and the tube envelope I8, although a glass envelope could be used if desired. The cathode and anod electrodes 22, Illa are preferably surrounded with a shield "connected to the cathode 22, to prevent current flow between the cathode 22 and the tube envelope IS.

A battery 28 or similar voltage source has one terminal connected to the tube envelope I6 (and, hence, to the anode IOa) through a load As long as the ends Ila. Ilb of the bar III are at rest, a uniform space current will flow from the cathode 22 to the anode Ila in the tube. However, if a sound wave having a frequency corresponding to the natural vibratory frequency (specifled above) for the bar I. impinges on the exposed end llb of the bar, the exposed bar end IOb will vibrate in the manner illustrated in Fig. 1, with the vibrations induced by the sound wave being transmitted between the bar ends by mechanical coupling within the bar, with the result that the spacing between the anode Ila and the cathode 22 will vary, producing corresponding variations in the current flowing in the tube and in the load impedance ll. To detect the arrival of a sound wave, the impedance Ill can be connected through a capacitor 32 to an amplifier and detector 24 which will amplify and detect or rectify the fluctuating voltage developed across the load 20, and the resultant output of the amplifier-detector 24 can be utilized to actuate any suitable indicator 36, such as a meter or the like. In the case of modulated sound wave, the indicator 2! could be a headset, loudspeaker, or similar transducer.

It should be noted that the vibratory motion illustrated by the lines Ila, Ilb in Fig. 1 is not strictly accurate as regards the conditions at the center of the bar. That is to say, although the free bar ends Illa, lib will vibrate substantially in the manner shown, there will be little or no motion at the center of the bar. In this regard, an

electronic tube constructed and operated in accordance with the invention represents a material improvement over prior art movable electrode tubes wherein one wall of the tube comprises a flexible diaphragm or other movable structure permitting transfer .of mechanical motion through the tube wall. In my improved tube, it is only necessary that the tube envelope wall be sufllciently thin'at the point of bar support so that the envelope wall will not interfere with or damp the motion of the bar on either side of the center thereof. For example, in a typical case, the free-free bar I. may be 2.54 cm. long and .1 cm. in thickness, and the supporting wall of the tube approximately .2 cm. thick and approximately 1 cm. in diameter. The bar III may be composed of a material having a ratio n equal approximately to 3X10". Suitable materials ,are stainless steel, berylium, or various aluminum alloys such as dural.

In the foregoing example, the frequency of maximum response for the transducer is approximately 26 kilocycles.

In Fig. 1, the bar I. is shown asbeing flat rather than rectangular in shape. A flat bar is deemed preferable in many instances since its principle plane of vibration can be oriented readily to insure that the bar end Illa will always move toward and away from the cathode 22 rather than laterally with respect thereto, thus insuring maximum current change in the tube. At the same time, it will be understood that a flat bar will give the transducer a bidirectional response pattern with the plane of maximum response being at right angles to the bar face. If a more nearly omnidirectional response pattern is preferred, the bar can be made with a rectangular, circular, or other similar cross section, although the response pattern will still exhibit a slight maxima for sound waves arriving along a line through the. cathode 22 and the bar anode IOa since motion of the bar end directly toward and away from the cathode 22 will cause greater current fluctuation than transverse displacement.

It is also to be understood that the invention is not limited to a diode-type tube, nor to the particular form of diode illustrated wherein the free bar end inside the tube serves as an anode,'

son. In any event, it will be seen that ther are I several ways in which a free-free bar can be used as a frequency sensitive element for transmitting mechanical motion through the envelope wall of an electronic tube in accordance with my invention.

The sensitivity of a transducer constructed in accordance with the invention can be increased by providing a baflle adjacent the exposed end of the free-free bar to decrease the adverse eflects caused by a sound wave acting simultaneously on opposite surfaces of theexposed bar end. In Fig. 3. for example, I have shown the upper portion of a tube envelope I8 with one end IOb of a free-free bar extending therefrom. A generally U-shaped plate member 28 is mounted on the top of the tube envelope I6 adjacent to and coplanar with the free bar end Illb (when the latter is at rest). The baflle plate 28 effectively increases the free path distance between opposite faces of the free bar end IIIb so that a sound wave impinging on one of the faces must travel an ap-.

preciable distance before reaching the opposite face and will. therefore, have less tendency to interfere with the vibrations originally set up in the bar I0.

Fig. 4 shows a slightly different type of baille 38a in the general form of a longitudinal section of a cylinder partially enclosing the free bar end I 0b. The form of baille shown in Fig. 4 has the same effect as that shown in Fig. 3, with the additional feature that the transducer shown in Fig. 4 will be, for the most part, unresponsive to sound waves approaching the free bar end IOb on the enclosed side thereof. Accordingly, this type of bailie will provide a unidirectional transducer, and when mounted for pivotal movement about the longitudinal axis thereof, can be utilized in sound wave direction finding equipment or the like.

I claim:

1. A sound wave transducer responsive to sound waves of predetermined frequency for generating alternating electrical currents corresponding in frequency to said predetermined frequency, said transducer comprising an evacuated envelope containing a plurality of electrodes for developing space current flow in said envelope, one of said electrodes comprising a vibratory free-free bar extending through a wall of said envelope and supported at its center by said wall so that one end of said bar is disposed outside of said envelope and the other end of said bar is disposed inside of said envelope in a position to alter said electron stream due to movement of said other bar end, said free-free bar comprising an elongated vibratory element supported approximately at its center and adapted to vibrate transversely at a frequency determined by the bar dimensions with a nodal point at the bar center and antinodal points at or near the bar ends when one bar end is set in motion transversely, the physical dimensions of said bar being such that said bar is mechanically vibratably resonant at said predetermined frequency.

2. In a sound wave transducer, a space discharge device comprising and envelope containing a plurality of electrodes for developing space current flow within said envelope, one of said electrodes comprising a vibratory freefree bar extending through a wall of said envelope and supported by said envelope wall intermediate the ends of said bar, said free-free bar comprising an elongated vibratory element supported approximately at its center and adapted to vibrate transversely at a frequency determined by the bar dimensions with a nodal point at the bar center and antinodal points at or near the bar ends when one bar end is set in motion transversely, said bar being vibrationally responsive to the impingement of sound waves of predetermined frequency on the free bar end outside of said envelope to cause the free bar end inside of said envelope to move with respect to another of said electrodes.

3. In a sound wave transducer as defined in claim 2, bafile means mounted on said envelope wall adjacent said free bar end outside of said envelope to increase the free space path from one side of said outside free bar end to a side opposite said one side.

4. In a sound wave transducer as defined in claim 2, baflie means mounted on said envelope wall adjacent to and partially surrounding said free bar end outsideof said envelope to render said transducer predominantly responsive to sound waves approaching said device on the exposed side of said outside free bar end.

5. In a sound wave transducer, an electronic tube comprising an evacuated envelope containing a plurality of electrodes, one of said electrodes comprising a vibratory free-free bar exquency on the free bar end outside of said envelope to cause the free bar end inside of said envelope to move with respect to another of said electrodes.

6. A vacuum tube comprising an evacuated envelope, a vibratory free-free bar extending through said envelope, said free-free bar comprising an elongated vibratory element supported approximately at its center and adapted to vibrate transversely at a frequency determined by the bar dimensions with a nodal point at the bar center and antinodal points at or near the bar ends when one bar end is set in motion transversely, a cathode electrode within said envelope, the free end of said bar inside of said envelope being adjacent to said cathode and constituting an anode electrode to cooperate with said cathode electrode in establishing space current flow within said envelope.

7. A sound wave transducer comprising an evacuated envelope, avibratory free-free bar extending through one wall of said envelope and supported at its midpoint by said wall, said freefree bar comprising an elongated vibratory element supported approximately at its center and adapted to vibrate transversely at a frequency determined by the bar dimensions with a nodal point at the bar center and antinodal points at or near the bar ends when one bar end is set in motion transversely, the portion of said bar inside said envelope constituting a first electrode within said envelope, a second electrode rigidly mounted within said envelope to cooperate with said first electrode in establishing space current fiow within said envelope, and a shielding electrode mounted within said envelope surrounding said first and second electrodes and connected to said second electrode.

LESLIE J. ANDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Norton Mar. 29, 1949 Number ducer, by Harry F. Olsen, from Tele-Tech, July 1947, pages 66-67, effective date of article March 1947. 

